Calcium phosphate based compounds, in particular hydroxyapatite, have excellent biocompatibility and find increasing use as implant materials including artificial tooth roots and bones. With a view to exploiting the high biocompatibility of calcium phosphate based compounds, attempts are also being made to use them as packings for liquid chromatography for separation of proteins, enzymes, etc.
The particles of calcium phosphate based compounds useful as packings for liquid chromatography can be produced by synthesizing a slurry of calcium phosphate by a wet method, spray drying the slurry to form particles, and firing the particles at a predetermined temperature. The lower the firing temperature, the higher the ability of the particles to adsorb proteins and other solutes but the lower the strength of the particles. On the other hand, if the firing temperature is increased, the adsorption capacity is lowered but the strength of the particles is improved.
A major problem with liquid chromatography columns that are packed with calcium phosphate based compounds is that when they are subjected to repeated use and the flow rate of the feed solution increases, a larger volume of voids form in the inlet portion of the column where the solvent stream is injected, thereby causing deterioration of the column performance. Two possible causes of this phenomenon are (1) dissolution of calcium ions into the solvent, and (2) mechanical impact caused by the supply of feed solutions. Because of these reasons, a column packed with particles of a calcium phosphate based compound that have been fired at a low temperature lacks durability in spite of its high capacity for adsorbing proteins and other solutes. On the other hand, a column packed with particles of a calcium phosphate based compound that has been fired at a high temperature suffers the problem of reduced adsorption capacity in spite of high durability.